Alkaline dry batteries have an inside-out structure, including a positive electrode case also functioning as a positive electrode terminal, cylindrical positive electrode material mixture pellets disposed to closely contact the positive electrode case, and a gelled zinc negative electrode disposed in the center of the positive electrode pellets with a separator interposed therebetween. The positive electrode material mixture is mainly composed of manganese dioxide. With recent spread of digital devices, an electric load of those devices in which these batteries are used has been increasing gradually, leading to a demand for batteries excellent in high-load discharge performance.
Responding to the demand, Patent Document 1 proposed mixing nickel oxyhydroxide in the positive electrode material mixture for a battery excellent in discharge performance under high-load. Nowadays, such batteries including nickel oxyhydroxide in the positive electrode material mixture are in actual use and are widespread.
For nickel oxyhydroxide for the above alkaline battery, generally, a spherical or egg-shaped nickel hydroxide that has been used for alkaline storage batteries, as in Patent Document 2, is oxidized with an oxidizing agent such as a sodium hypochlorite aqueous solution for the usage. For the raw material spherical nickel hydroxide, β-type with a high bulk density (tap density) is used, and this is converted to β-type spherical nickel oxyhydroxide by a treatment with an oxidizing agent, aiming for a further dense filling of the nickel oxyhydroxide in the battery. At this time, for the purpose of increasing the positive electrode capacity (utilization) in the battery, as shown in Patent Document 3 for usage in alkaline storage batteries, a nickel hydroxide solid solution containing cobalt, zinc, and the like in its crystal under solid-solution state is sometimes used as a starting material. Recent proposals for nickel oxyhydroxide used in primary batteries include spherical one (Patent Document 4), a solid solution including zinc (Patent Document 5), and a solid solution including zinc and cobalt (Patent Document 6). Those are basically application of known technologies of positive electrode material for alkaline storage battery as noted in the above to usage in primary batteries.
Such alkaline batteries in which nickel oxyhydroxide is mixed into the positive electrode material mixture have significant problems: storage characteristics are poor and especially when stored under high temperatures, self-discharge in positive electrode is significant compared with those alkaline batteries not including nickel oxyhydroxide. For improvement in view of such problems as well, technologies of alkaline storage battery are largely applied. For example, there is proposed retarding the self-discharge by adding ZnO and Y2O3 to positive electrode material mixture (Patent Document 7), and there is proposed retarding the self-discharge by adding an oxide of rare earth elements such as Yb2O3 and Er2O3 to the positive electrode material mixture (Patent Document 8). Also, since nickel oxyhydroxide with a higher valence (γ-type structure) shown in Patent Documents 9 and 10 tends to have a lower oxidation-reduction potential than β-type, as an improvement in storage characteristics different from those of Patent Documents 7 and 8, a method by using such nickel oxyhydroxide with a higher valence may be considered. Usage of such nickel oxyhydroxide with a higher valence retards the decomposition reaction of water (oxygen-generative reaction) in the positive electrode, and improves storage characteristics.
[Patent Document 1] Japanese Laid-Open Patent Publication No. Sho 57-72266
[Patent Document 2] Japanese Examined Patent Publication No. Hei 4-80513
[Patent Document 3] Japanese Examined Patent Publication No. Hei 7-77129
[Patent Document 4] Japanese Laid-Open Patent Publication No. 2002-8650
[Patent Document 5] Japanese Laid-Open Patent Publication No. 2002-75354
[Patent Document 6] Japanese Laid-Open Patent Publication No. 2002-203546
[Patent Document 7] Japanese Laid-Open Patent Publication No. 2001-15106
[Patent Document 8] Japanese Laid-Open Patent Publication No. 2002-289187
[Patent Document 9] Patent Publication No. 3239076
[Patent Document 10] Domestic Re-publication of International Publication No. WO 97/19479